I'm wanting to install a water softener. Can I merge the drain pipe from the softener with the line coming from the relief valve on the top of the water heater? I'm concerned about the elevation of where I would need to merge the pipe, in respect to the exit port on the softener.
The down pipe from your water heater relief valve should not be plumbed into a drain. the output end may be hovering over a floor drain but not hard plumbed to drain lines. If the relief blows off due to high pressure (possibly steam) it could rupture a hard plumbed drain line.
There are two drain lines from a water softener; one over-flo from the brine tank and one off the control valve. The over-flo line could go to the same place as the heater relief output. The line from the control valve will have a significant pressure while the softener is in recharge or backwash modes and should be hard plumbed through a check valve to your drain piping to sewer or septic.